This invention relates to a method for preoperative planning of surgery. More particularly, it pertains to a method of preoperative planning of a bone cut and joint replacement using radiant energy scan imaging to determine the position of a bone-cut-defining guide relative to the bone to be cut.
The preferred method of the present invention is for the replacement of a total knee. This includes the removal of bone sections from the distal femur and proximal tibia for replacement by a knee joint prosthesis associated with each of these bones.
Total knee replacement is a common orthopaedic surgical procedure currently performed over 150,000 times each year in the U.S. The clinical results of many operations are excellent with complete relief of pain, improvement in function, restoration of motion, and correction of deformity in over 90% of the cases. However, there are a number of cases in which failures occur following the knee replacement. One of the most important causes for failure of the procedure is from prosthesis component loosening because of unbalanced loading of the tibial component caused by improper knee joint alignment. Because of this fact, all total knee implantation systems attempt to align the reconstructed knee joint in the mechanical axis in both the coronal and the sagittal planes. If achieved, this results in the placement of the total knee prostheses in a common mechanical axis which correspondingly is highly likely to produce a successful long-term result.
Reproducing the mechanical axis at surgery is presently done by one of two different techniques, which use either the external bone landmarks at the hip and ankle joints or the medullary canal of the femur or a combination of these two systems for alignment. Knee systems which use the center of the femoral head as a landmark for orienting the femoral component require an operative radiograph of the hip joint to position an external marker for alignment of the femoral cutting guide. Intramedullary knee systems require a preoperative radiograph of the femur in order to determine the angle between the anatomical and the mechanical axes of the femur for proper orientation of the femoral cutting guide. These intramedullary systems require the surgeon to estimate the placement of a drill hole into the distal femur at a central location in the bone for introduction of a small diameter rod into the medullary canal to produce the correct component alignment. The proximal tibia is cut perpendicular to the mechanical axis of the tibia by adjusting the tibial cutting guide in relation to the knee and ankle joints. Both of these techniques necessitate intraoperative visual estimation of the location of the midpoints of the distal femur, the proximal tibia and the ankle joint by the surgeon. The alignment of the components in the sagittal plane is also done by visual means or the "eyeball" method.
In summary, with the present total knee instrument systems, correct knee alignment involves the following:
1. preoperative determination of the angle between the anatomical and mechanical axes of the femur from the radiographs, and appropriate placement of the medullary rod entrance hole in the femur for the intramedullary system of femoral component alignment; PA0 2. localization of the center of the femoral head by external markers after operative radiographs are taken, and correct estimation of the center of the distal femur for the external alignment system of femoral alignment; PA0 3. visual estimation of the centers of the proximal tibia and of the ankle joint in both the coronal and sagittal planes for correct tibial component alignment.
These alignment techniques may produce error from the fact that the surgeon must estimate the correct position of all bone landmarks and from inaccuracies in the preoperative radiographs of the knee joint. Flexion contractures of the knee will cause significant errors in the tibiofemoral angle (the angle between the femoral anatomical and mechanical axes). Medullary systems require accurate placement of the entrance hole for the alignment rods since the rod does not tightly fit into the medullary canal and may be angled into it if the drill hole is placed too far medially or laterally. A considerable amount of the operative time in total knee replacement surgery is expended in positioning and attaching the alignment instruments and in double-checking their placement, which is essential since any system may fail and have to be overruled by the experienced eye of the surgeon.
The present invention overcomes the inherent inaccurateness of the presently used systems by combining several steps. Selected regions of the body adjacent a bone to be resectioned are scanned with radiant energy to obtain representations of the regions for defining the structure of the specific bone and adjacent body regions. From the representations, desired positions of a cutting guide relative to the bone are determined. Thus, by having these specific features of bone structure identified and used for determining specific placement of the cutting guide, accurate and precise placement during the surgical procedure is provided.
Control of the guide surface of a cutting guide which defines the contour of a desired bone cut is assured where the cutting guide includes one or more gauge members positionable adjacent a selected position on the bone and adjustable relative to the guide surface for positioning the guide relative to the bone. These specific settings of the gauge members relative to the guide surface are determined from the representations of the selected body regions adjacent and including the bone having a section to be replaced. In the preferred method of the present invention, a joint is replaced and the replacing prostheses are aligned relative to axes associated with each joint-forming bone so that the resulting prostheses will have a specific alignment relative to those axes. By determining the position of the gauge member relative to the axis, the position of the cutting guide surface is established prior to the surgical procedure, with corresponding precise placement of the guide during the procedure.
It should be noted that CT scan information has been used in the past relative to prostheses. For example, in an article in Volume 97 (June 1979) of Fortschritte der Medizin on page 781-784 entitled "Ein neues Verfahren zur Herstellung Alloplastischer Spezilimplantate fur den Becken-Teilersatz", a method of preparing alloplastic implants is described in which a three-dimensional model of a patient's pelvis is constructed by assembling styrofoam sheets made from computer tomography films. U.S. Pat. No. 4,436,684 assigned to the same assignee as this application, describes using information obtained from CT scans to drive a sculpting tool to make a corporeal model.
As will be more apparent hereinafter, with the present invention a surgeon's need to do preoperative planning from plain radiographs is eliminated. Because there is no need to determine the placement or adjustment of cutting guides at the time of surgery, fewer instruments are necessary and the surgical procedure is simplified and shortened. Accurate sizing of the prosthesis components is possible by measurement of the axial CT scan slices at the level of component placement for each bone. The vast majority of all important intraoperative decisions are decided preoperatively by this intensive and precise planning method. The surgeon has to make fewer critical judgment calls during surgery and is able to eliminate the constant visual monitoring of the alignment instrument. Elimination of these steps markedly reduces the operative time of the procedure.